1. Field of the Invention
This invention relates to a cruise control system for a vehicle which automatically maintains a desired vehicle speed, and more particularly to such a cruise control system for an automatic transmission vehicle.
2. Description of the Prior Art
Recently, there has been put into practice a cruise control system which automatically keeps the vehicle speed at a desired speed set by the driver. In the control system, when the vehicle speed reaches a desired speed, the driver operates a setting means, and thereafter the throttle valve of the engine is automatically controlled according to the difference between the desired speed and the actual vehicle speed to maintain the desired speed. An example of such a cruise control system is disclosed in Japanese Unexamined Utility Model Publication No. 57(1982)-164228.
In such a cruise control system, when the required driving force is increased, for example, as the vehicle ascends a slope during constant speed cruising, the throttle opening is generally increased to increase the amount of fuel to be fed to the engine in order to prevent deceleration. However, when the slope is too steep for the desired speed to be maintained by increasing the amount of fuel to be fed to the engine, the transmission is automatically shifted down.
When the transmission is shifted down, the driving force transmitted to the wheels is increased and the vehicle speed can be increased to the desired speed. However, the downshift of the transmission abruptly increases the driving force transmitted to the wheels and possibly involves the following problems. First, a shock can be generated by the abrupt increase of the vehicle speed to the desired speed by the increased driving force. Second, if downshift is effected during cornering, the increased driving force can increase the vehicle speed abruptly, thereby increasing the centrifugal force acting on the vehicle body, forcing the vehicle outwardly. This adversely affects driving safety.